1. Field of Invention
The present invention relates to an Fe--Ni alloy with improved blanking property, which is suitable for the electrode material of an electron-gun. The present invention also relates to press-blanked parts of an electron gun, which are manufactured by blanking the Fe--Ni alloy and by piercing minute apertures though which an electron beam is passed.
2. Description of Related Art
FIG. 1 is a cross-sectional view of the known shadow-mask type color cathode-ray tube. A fluorescent coating 2 is applied on the panel 1 and emits three primary colors, i.e., red, green and blue. An electron gun 4 is provided in the neck of the cathode-ray tube and emits electron beams 3. The electron beams 3 are deflected and scanned by the deflecting yoke 5. Reference numerals 6 and 7 denote the shadow mask and the magnetic shield, respectively. All of these parts 1 through 7 are known.
FIG. 2 illustrates a rolled and then press-blanked part 10, which is an electrode of the electron gun 4. Reference numerals 10r, 10r ' denote the major rolled surfaces. FIGS. 2 (A) and (B) are the illustrative view and cross-sectional view of an electrode, respectively. The minute apertures 10a, 10b and 10c are formed by coining and blanking a sheet and allow the respective electron beam to pass therethrough for generating the colors red (10a), green (10b) and blue (10c). For the parts of an electron-gun used in an image-receiving tube, a non-magnetic stainless steel sheet having a thickness of from 0.05 to 0.5 mm has conventionally been used, since the non-magnetic property is important for the electrodes 10 which accelerate the thermal electrons emitted from the cathode of the electron gun. Such a sheet is intermediately coined and then blank-worked as above or directly blanked without coining.
Recently, rather than the non-magnetic property, a low thermal expansion property is considered to be more important, because the image-receiving tube of a computer-display or the like has become highly precise and its performance is enhanced. In this case, picture performance (purity of color) is influenced by subtle dimensional changes in the electrode 10 due to its thermal expansion.
Since an Fe--Ni alloy, particularly Fe-42% Ni alloy (the so-called 42 Alloy), has a low thermal expansion property, its use for the electrode material has now been undertaken. However, when the conventional 42 Alloy is blanked by means of a punch to form the electrode part, burrs are detrimentally formed at the front end 10e (FIG. 3), where the blanking tailings are separated from the sheet. The blanking burrs exert a detrimental effect upon the dimensional accuracy of the electron-gun part which is required to be highly precise. In addition, since an abnormal discharge occurs at the burr, when a high voltage is applied to the electrode, the voltage-resistance of an electron-gun is lowered. This is a very serious disadvantage. Lessening of the burrs of an electrode-gun part will be more and more highly demanded along with precision of an image-receiving tube.
There are, heretofore, proposals in Japanese Unexamined Patent Publications Nos. 6-122,945, 6-184,703, 7-84,199, 7-3,400 and 7-34,200 to improve the blanking property of an Fe--Ni alloy. In Japanese Unexamined Patent Publication No. 6-184,703, the S content of an Fe--Ni alloy is limited to a range of from 0.002 to 0.05%, and the sulfur or sulfur compound is dispersed along the grain boundaries or within the crystal grains. However, the burrs cannot be satisfactorily prevented only by means of adding a specified amount of sulfur and hence efforts to improve the machinability, when the alloy is subjected to the extremely precise press forming, have been carried out in recent years.
Next, the proposals made in Japanese Unexamined Patent Publications Nos. 6-122,945, 7-84,199, and 7-34,200, reside in that such strengthening elements as Ti, Nb, V, Ta, W, Zr and the like are added to enhance the hardness and hence to induce a degree of brittleness thereby suppressing the burrs. The life of a metal die is, however, shortened due to the increased hardness of the Fe--Ni alloy, and the cost of the alloy must increase due to addition of special elements.
Japanese Unexamined Patent Publication No. 7-34,200 discloses a lead-frame made of an Fe--Ni alloy, and proposes to specify the morphology and the number of non-metallic inclusions, of which the length is 1 .mu.m or more measured in the direction parallel to the rolling direction. That is, the number of non-metallic inclusions in a cross section of 10 mm.sup.2 parallel to the rolling direction is limited. As a result of such specification and limitation of the inclusions, it is allegedly possible to increase the number of pins of a lead-frame. As is described in this patent publication, it is necessary to specify the maximum length of non-metallic inclusions depending upon the shape, dimension and the like of the lead-frame to be produced. This description suggests that there is a relationship between the appropriate morphology of non-metallic inclusions and a particular part of an electronic device. More specifically, it is understood that the morphology of non-metallic inclusions appropriate for a part of an electron-gun must be different from that appropriate for a lead frame, because shapes and dimensions of the parts are different. However, such relationship has heretofore not been clarified.